The applicant is aware of processes for the synthesis of water from a carbonaceous feedstock, such as natural gas and coal, which processes also produce hydrocarbons.
One such process is the Fischer-Tropsch process of which the largest product is water and, to a lesser extent, hydrocarbons including olefins, paraffins, waxes, and oxygenates. There are numerous references to this process such as, for example on pages 265 to 278 of “Technology of the Fischer-Tropsch process” by Mark Dry, Catal. Rev. Sci. Eng., Volume 23 (1&2), 1981.
The products from the Fischer-Tropsch process may be processed further, for example by hydroprocessing, to produce products including synthetic crude oil, olefins, solvents, lubricating, industrial or medicinal oil, waxy hydrocarbons, nitrogen and oxygen containing compounds, motor gasoline, diesel fuel, jet fuel and kerosene. Lubricating oil includes automotive, jet, turbine and metal working oils. Industrial oil includes well drilling fluids, agricultural oils and heat transfer fluids.
In certain areas where carbonaceous feedstocks are to be found, water is in short supply and a relatively costly commodity. Also, environmental concerns prevent the dumping of polluted water derived from the Fischer-Tropsch process into natural water ways and the sea thereby presenting a case for the production and recovery of useable water at the source of the carbonaceous feedstocks.
The carbonaceous feedstocks typically include coal and natural gas that are converted to hydrocarbons, water and carbon dioxide during Fischer-Tropsch synthesis. Naturally, other carbonaceous feedstocks such as, for example, methane hydrates found in marine deposits can also be used.
Before the water produced during the Fischer-Tropsch process is purified in accordance with the present invention, it is typically subjected to preliminary separation aimed at isolating a water-enriched stream from the Fischer-Tropsch products.
The preliminary separation process includes condensing the gaseous product from the Fischer-Tropsch reactor and separating it in a typical three-phase separator. The three streams exiting the separator are: a tail gas, a hydrocarbon condensate including mainly hydrocarbons in the C5 to C20 range and a reaction water stream containing dissolved oxygenated hydrocarbons and suspended hydrocarbons.
The reaction water stream is then separated using a coalescer that separates the reaction water stream into a hydrocarbon suspension and a water-rich stream.
The coalescer is capable of removing hydrocarbons from the reaction water stream to a concentration of between 10 ppm and 1000 ppm, typically 50 ppm.
The water-enriched stream thus obtained forms the feedstock for the method according to the present invention and will be denoted in this specification by the term “Fischer-Tropsch reaction water”.
The composition of the water-enriched stream or reaction water is largely dependent on the metal used in the Fischer-Tropsch catalyst and the reaction conditions (e.g. temperature, pressure) employed. The Fischer-Tropsch reaction water can contain oxygenated hydrocarbons including aliphatic, aromatic and cyclic alcohols, aldehydes, ketones and acids, and to a lesser extent aliphatic, aromatic and cyclic hydrocarbons such as olefins and paraffins.
The Fischer-Tropsch reaction water may also contain small quantities of inorganic compounds including metals from the Fischer-Tropsch reactor, as well as nitrogen and sulphur containing species that originate from the feedstock.
The influence of the type of Fischer-Tropsch synthesis used on the quality of Fischer-Tropsch reaction water is illustrated in typical organic analysis (Table 1) of Fischer-Tropsch reaction water generated from three different synthesis operating modes, namely:
TABLE 1Typical Organic Composition of Fischer-Tropsch reaction waterfrom Different Fischer-Tropsch Synthesis Operating ModesComponentLTFTLTFTHTFT(mass %)(Cobalt Catalyst)(Iron Catalyst)(Iron Catalyst)Water98.8495.5494.00non-acid oxygenated1.0753.8304.80hydrocarbonsAcidic oxygenated0.0750.6101.20hydrocarbonsOther Hydrocarbons0.020.020.02Inorganic<0.005<0.005<0.005components
It is evident from the typical analyses of Fischer-Tropsch reaction waters of different origin (Table 1) that these waters, in particular HT Fischer-Tropsch reaction water, contain relatively high concentrations of organic compounds, and direct application or disposal of these waters is generally not feasible without further treatment to remove undesirable constituents. The degree of treatment of the Fischer-Tropsch reaction water depends largely on the intended application, and it is possible to produce a wide range of water qualities ranging from boiler feed water to partially treated water which may be suitable for discharge to the environment.
It is also possible to co-treat Fischer-Tropsch reaction water with other typical process waste water as well as rain water.
The water purification processes described in this invention may, after making minor adaptations, also be used for the processing of aqueous streams derived from generic synthesis gas conversion processes using metallic catalysts similar to the catalysts used during Fischer-Tropsch synthesis.